The present invention relates generally to open-cell foams and more particularly to carbon nanotube foams.
Structural foams have a variety of applications in modern society, such as in construction, energy dissipation, cushioning, and packaging. See L. J. Gibson and M. F. Ashby, Cellular Solids, Structure and Properties (Pergamon, New York, 1997); N. C. Hilyard & A. Cunningham, Low Density Cellular Plastics, Physical Basis of Behavior (Chapman and Hall, London, UK, 1994). The struts between adjacent cells form the architecture of a foam, and it is the bending and buckling of these struts that allow the foam to be compressed. The properties of the struts (e.g., composition, geometry, dimension) dictate the compressive behavior of foams. See J. H. Kinney et al., “Three-dimensional imaging of large compressive deformations in elastomeric foams,” J. Appl. Poly. Sci. 80, 1746-1755 (2001); H. X. Zhu et al., “Analysis of the high strain compression of open-cell foams,” J. Mech. Phys. Solids 45, 1875-1904 (1997).
Compressive strength and compressibility are two important factors that determine the performance and applications of foams. These two factors, however, are of opposing nature. By increasing a foam's volume fraction of cells (i.e., void area between struts), the foam's compressibility is increased while the strength is decreased. See Hilyard & Cunningham (1994); D. Klempner & K. C. Frisch, Handbook of Polymeric Foams and Foam Technology (Hanser, New York, 1991), ch. 4, 6, 9; H. X. Zhu et al., “Analysis of the elastic properties of open-cell foams with tetrakaidecahedral cells,” J. Mech. Phys. Solids. 45, 319-343 (1997). For a foam at a fixed chemical composition, its modulus (Ef) decreases with increasing relative cell volume (φ) as Ef=CE(1−φ)2, where C is a constant (close to unity), and E is the cell edge modulus. See Hilyard & Cunningham (1994). Although metallic foams, such as Al foams, have a relatively higher compressive strength than polymeric foams, metallic foams have poor resilience upon load release due to permanent deformation of the metallic cell structure. See L. J. Gibson, “Mechanical behavior of metallic foams,” Annu. Rev. Mater. Sci. 30, 191-227 (2000). Thus, there currently exists a need in the art for structural foams with high compressive strength, compressibility, and resilience.
Individual carbon nanotubes possess exceptional mechanical strength, low density, and high elasticity. See M. S. Dresselhaus et al., Science of Fullerenes and Carbon Nanotubes (Academic, San Diego, 1996); R. H. Baughman et al., “Carbon nanotubes—the route toward applications,” Science 297, 787-792 (2002); D. Qian et al., “Mechanics of carbon nanotubes,” Appl. Mech. Rev. 55, 495-533 (2002). For example, an individual nanotube exhibits extreme structural flexibility and can be repeatedly bent through large angles and strains without structural failure. See supra Qian et al. (2002). See S. Iijima et al., “Structural flexibility of carbon nanotubes,” J. Chem. Phys. 104, 2089-2092 (1996); V. Sazonova et al., “A tunable carbon nanotube electromechanical oscillator,” Nature 431, 284-287 (2004); M. R. Falvo et al., “Bending and buckling of carbon nanotubes under large strain,” Nature 389, 582-584 (1997). The ability of an individual nanotube to adopt and switch between various buckled morphologies makes it capable of accommodating and sustaining large local strains while maintaining structure integrity. See B. I. Yakobson et al., “Nanomechanics of carbon tubes: instabilities beyond linear response,” Phys. Rev. Lett. 76, 2511-2514 (1996); O. Lourie et al., “Buckling and collapse of embedded carbon nanotubes,” Phys. Rev. Lett. 81, 1638-1641 (1998). However, to create a carbon nanotube foam for real-world applications, a scalable method is needed to produce a foam containing many nanotubes that collectively contribute to the foam's unique properties up to macroscale levels.